Throughout this document the term “frame” will denote the sequence of symbols that includes the useful part of the transmitted data. To correctly demodulate a received signal, a receiver must recognize the beginning of the useful part. Typically, the transmitter inserts before the data bits two pre-determined bit-sequences referred as “preamble” and “unique word” (UW). The preamble sequence is mainly used for symbol, clock synchronization or carrier recovery purposes, while the UW is used to obtain frame synchronization.
Recognition or detection of a known sequence of bits contained in the received stream is generally carried out via a correlator. Typically, the correlator is hard input based, although also soft input based correlator approaches exist. In the case of a hard correlator, the received signal, after passing through an analog front end (AFE) that includes, among others, an amplifier and an analog to digital converter (ADC), is hard demodulated and correlated with the pre-stored UW: if the output of the correlator exceeds a pre-determined correlation threshold, frame synchronization is declared. A problem of such a scheme is that if the chosen correlation threshold is too high, i.e. if a large number of matches is required between the pre-stored UW and the received signal, the probability of missed detection in the presence of noise is high. Conversely, if the correlation threshold is too low, the probability of false detection grows and false alarms are generated with high frequency. For example, see W. Schrempp and T. Sekimoto, “Unique word detection in digital burst communications,” IEEE Transactions on Communication Technology, vol. 16, pp. 597-605, August 1968.
Several strategies have been conceived in the past by the research community to limit the missed and false UW detections. For instance, methods to generate UW bit sequences with a pre-fixed length L and good auto-correlation and cross-correlation properties have been devised, such as discussed in M. Chen, K. C. Chua and Q. L. Ding, “A fast algorithm for choosing frame synchronization unique word,” IEEE GLOBECOM 98, vol. 6, pp. 3437-3442, November 1998. Furthermore, among the techniques focusing on the computation of the correlation, the patent by Dabak (A. J. Dabak, “Frame synchronization with Unique-Word dependent filter coefficients,” U.S. Pat. No. 6,480,559, Nov. 1998), in which a method is disclosed for reducing the “sidelobes” of the correlation output by substituting, at the receiver side, the pre-stored UW with a sequence that is mathematically derived from the original UW and the preamble. Also, the article by Sasai et al. (H. Sasai, M. Takeyabu, K. Minomo, F. Takahata and J. Ogikubo, “Unique word detection using quantized soft-decision data,” Electronics and Communications in Japan, vol. 77, pp. 80-93, March 2007), suggests to substitute the hard input based correlator with a soft one: in this case, after the AFE, the Euclidean distance between the pre-stored modulated UW and the soft values of the received sequence is computed. These approaches do not completely address the problem of limiting the missed and false UW detections.
A different approach (adopted for example in T. M. Mynett, B. A. R. Kobari and R. C. Hyers, “Method and apparatus for performing very fast message synchronization,” U.S. Pat. No. 5,870,444, April 1996) includes transmitting the same UW repeatedly. The use of repeated UWs clearly reduces the missed detections, since the receiver will probably detect at least one of the transmitted UWs by a correlation process. By checking the presence of more than one UW, also the false detections are limited. Moreover, the repetition of the same UW sequence, instead of increasing the UW length, limits the dimension of the correlator at the receiver. However, the achievements of this technique are obtained at the cost of a transmission overhead that may be undesirable in many applications.
Another method that introduces redundancy in transmission coding, see P. Febvre and P. Fines, “Communication method and apparatus”, WO 02/062005, August 2002, where, at the transmitter, the UW is coded using a systematic code. Due to the systematic nature of the code, the UW keeps its good auto-correlation properties. At the receiver, the UW detection is divided into two steps. In the first step, a classic correlation detector computes the correlation between the received sequence and the stored UW. If a pattern is recognized to be the UW, the second step is performed: the receiver demodulates the received signal and uses its error correction power to determine if the output of the UW detector is a true or false detection. If the communication system uses a powerful error correcting code (for example a turbo code), false detections are reduced.
Besides the problem of the introduced redundancy, a drawback of this method is the need of coding the UW. In fact, if the transmission system does not include a coding block, a specifically designed code for the UW bits must be envisaged. Conversely, if the system allows coding for the data packet, certain codes should be adapted to include also the UW: for instance, in the case of turbo codes, the turbo interleaver length must be enlarged.
Another detection scheme for reducing false detections is adopted in many time division multiple access (TDMA) satellite systems. In TDMA satellite systems, the transmission is structured in frames that are consecutively transmitted. Each frame contains a sequence of a certain number of sub-frame fields generally referred as bursts. The bursts sequence is constituted by a first reference burst followed by station bursts associated to different ground stations. Each burst (reference or station) contains an identifying UW. When the first frame is received, the first UW detections are normally performed correlating the received sequence with the pre-stored UWs; for the successive frames, a time window may be formed around the expected time of arrival for the UWs such that the correlation detectors are only in operation for the window period (e.g. D. Roddy, “Satellite communications”, p. 450, McGraw-Hill 2006). This procedure does not avoid false detections during the first frame UW acquisition or implies an acquisition delay for verifying whether other UWs follow.
The idea of activating the UW correlator circuit depending on the estimated power of the received signal has been suggested in O. Nishimura, “Reception synchronization circuit, receiver using the same, and digital communication system,” U.S. Pat. No. 6,493,360, December 1998. This approach reduces false detections, since the correlator is switched off for a very long time. However, it implies the need of a circuitry with two thresholds that must dynamically evolve to obtain the desired missed and false detections. Moreover, the received signal power is not directly linked to the system performance, which determines the missed detection probability, thus, the threshold setup is not simple.